1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to an assembly structure of an electrode assembly, a terminal and a lead element of a secondary battery.
2. Description of the Related Art
Depending on the use or the power capacity of the battery, the secondary batteries can be classified into low power batteries (hereinafter referred to as “small batteries”) in which one to several battery cells are combined into a battery pack, and bulk size batteries for driving motors (hereinafter referred to as “large batteries”) in which several to tens of secondary battery cells are connected to form a battery pack.
Small batteries are used as the power source for various portable electronic devices, such as cellular phones, laptop computers, and camcorders. Large batteries are suitable for the power source of motor driven devices, such as hybrid electric vehicles.
Generally, small batteries have square and cylindrical shapes, when they are have only one cell. The small battery includes an electrode assembly in which a separator, serving as an insulator, is interposed between a positive plate and a negative plate of a belt shape. The separator and both plates are then spiral-wound. The electrode assembly is inserted into a cylindrical container to form the battery.
Conductive lead elements, which pass the current produced when the battery is operated, are attached to the positive and negative electrodes. The lead elements pass the current produced from the positive and negative electrodes to the positive and negative terminals.
When the above structure of the small battery is applied to the large battery, the dynamic kinetics necessary for large batteries are not satisfied in terms of capacity and power. Accordingly, secondary batteries have been provided including the multi-tap structure disclosed in Japanese Laid-open Patent No. 2003-7346 in which plural taps are attached to the electrode assembly. This secondary battery has a plurality of taps formed on the electrode assembly along one direction of the electrode assembly, and the taps are connected to external terminals.
However, such a multi-tap structure has a problem that needs a large workforce. Since a unit area of the tap in the tap type current collection structure is small, the battery has the limitation that it cannot satisfy the power characteristics required for the large battery.
Furthermore, considering the dynamic kinetics, since the large secondary battery needs a high energy density per unit volume, when the electrode assembly is mounted inside the container, it is necessary for the space between the electrode assembly and the container to be reduced as much as possible. This can maximize the volume of the electrode assembly occupying the inner space of the container having a fixed volume, thereby improving the capacity of the secondary battery.
However, when the electrode assembly is mounted inside the container, it is unavoidable for a gap between the electrode assembly and the container to be formed. This is caused by the tolerance of the manufacturing process and by the volume of the lead elements occupying the container not being reduced in size due to the structural limitations of the lead elements connected to the electrode assembly.